Soft-Story Effects on the Behavior of Fixed-Base and LRB Base-Isolated Reinforced Concrete Buildings

In Turkey, the entry stories of reinforced concrete (RC) structures are usually used for commercial purposes because they are slightly higher than other stories. Therefore, these RC structures sometimes exhibit what is called soft-story behavior. The best examples of this behavior are observed in the severe damage or collapse of RC structures with soft stories during the 1999 Marmara and Duzce earthquakes. To prevent this type of damage, seismic isolation systems can be used in the construction of new structures. This study focuses on the soft-story behavior of RC structures with fixed-base and lead core rubber bearing (LRB) systems under different seismic loads. The superstructure is modeled using the Turkish Seismic Regulation (TEC 2007). Because there is no detailed information concerning LRB isolators in the Turkish Seismic Regulations, the Uniform Building Code (UBC 97) is used to model the substructure system under consideration. In this study, four different structural models are designed to study the effect of the soft-story behavior. The dynamic analysis of the time domain on these frame systems is conducted using Ruaumoko software, and the influences of soft-story behavior in fixed-base and LRB base-isolated systems are investigated. The period of the frame system, story accelerations, inter-story drift ratio, base shear forces, and distribution of plastic hinges and their damage conditions are evaluated.